Milbemycin, or B-41, is a substance which is isolated from the fermentation broth of a milbemycin producing strain of Streptomyces. The microorganism, the fermentation conditions, and the isolation procedures are more fully described in U.S. Pat. No. 3,950,360 and U.S. Pat. No. 3,984,564. The structures of seven of the thirteen milbemycin compounds are described in said patents and the structures of all thirteen compounds are described in the Journal of Antibiotics 29 (6) June 1976 pages 76-35 to 76-42 and pages 76-14 to 76-16. The milbemycin compounds described in said patents do not have any carbohydrate groups substituted thereon.